Method and apparatus for controlling capacity of a multiple-stage cooling system

ABSTRACT

A method and apparatus for controlling the capacity of a multi-stage cooling system is disclosed. The system of the invention utilizes a plurality of commonly piped compressors having common suction load wherein at least one compressor being a variable speed compressor, means for establishing a single predetermined cooling stage &#34;cut-in&#34; pressure and a single predetermined cooling stage &#34;cut-out&#34; pressure, a plurality of condenser fans and a microprocessor based control means for controlling the operation of the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending U.S. patentapplication Ser. No. 062,390, now U.S. Pat. No. 4,825,662 filed June 15,1987, which is a continuation of U.S. patent application Ser. No.819,387 filed Jan. 16, 1986, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 706,403 filedFeb. 27, 1985, now U.S. Pat. No. 4,628,700, which is a continuation ofU.S. patent application Ser. No. 458,914 filed Jan. 18, 1983, nowabandoned, which is a continuation of U.S. patent application Ser. No.257,113 filed Apr. 24, 1981, now U.S. Pat. No. 4,612,776, which is acontinuation of U.S. patent application Ser. No. 062,525, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for increasing theaverage coefficient of performance of a multiple-stage refrigeration orcooling system.

In the past, the cycling of stages of a multiple-stage refrigeration orcooling system has been principally accomplished by setting each stageat a successively lower "cut-in" and "cut-out" pressure of therefrigerating fluid flowing in the suction line from the evaporator coilto the compressor(s) or cooling stages. The use of successively lower"cut-in" and "cut-out" pressure ranges for each cooling stage results inan average pressure which is lower than the mean pressure of thepressure differential between the "cut-in" and "cut-out" pressures ofthe highest stage. Various mechanical and electromechanical systems havebeen devised to attempt to solve this problem, primarily utilizing thesuccessively lower pressure ranges for each successive cooling stage asdescribed above.

Patents which are typical of the prior art for electromechanicallycontrolling refrigeration compressor capacity include the following U.S.Pat. Nos.: 3,885,938; 3,828,152; 3,719,057; 3,581,519; 3,580,006;3,552,137, and 3,377,816.

Increased efficiencies in a multi-compressor refrigeration system havingat least one variable speed compressor could be realized if each coolingstage could be controlled at a single highest "cut-in" and "cut-out"pressure levels that would ensure adequate temperatures in therefrigerated space served by the evaporator coils, and by enabling thecompressors to energize and deenergize when the variable speedcompressors have attained their respective maximum and minimum speeds.

SUMMARY OF THE INVENTION

At least one variable speed compressor is operated in parallel with aplurality of commonly piped compressors for controlling the capacity ofa multiple-stage refrigeration or cooling system. The variable speedcompressor is controlled over a range of speed to accommodate varyingsystem capacity load requirements, thus establishing an improved rangeof control and hence improve the coefficient of performance of thesystem.

Further improvement to the system performance is made by utilizingmultiple cooling fans to cool the condenser coil. By monitoring thetemperature of the condensed coolant or by monitoring the pressureinside the condenser coil, the number of fans cooling the condenser coilare changed and at least one variable speed fan is used to obtain afiner degree of control. Controlling the temperature or pressure of thecondenser within predetermined limits improves the coefficient ofperformance for the system.

A microprocessor-based control system responsive to system parameters isutilized. Suction pressure ranges which control the energization anddeenergization of compressors, control the speed of the variable speedcontrol compressors, and the desired time delays for improving the lifeof the compressors are programmed into the microprocessors.

The temperature of the system environment is continually monitored. Themicroprocessor control system is programmed to adjust the suctionpressure range of the system to maintain a cooled environment at thedesired temperature.

BRIEF DESCRIPTION OF DRAWINGS

In order that the manner in which the above-recited advantages andfeatures of the invention are attained can be understood in detail, amore particular description of the invention may be had by reference tospecific embodiments thereof which are illustrated in the appendeddrawings, which drawings form a part of this specification. It is to benoted, however, that the appended drawings illustrate only typicalembodiments of the invention and therefore are not to be consideredlimiting of its scope for the invention may admit to further equallyeffective embodiments.

In The Drawings:

FIG. 1 is a simplified schematic of a multiple-stage cooling orrefrigeration system including the capacity control apparatus accordingto this invention.

FIG. 2 is a block diagram schematic utilizing a microprocessor forcontrolling the refrigeration system illustrated in FIG. 1.

FIG. 3 is a flow chart illustrating the operation of the microprocessorbased control system of FIG. 2.

FIG. 4 is a graphical representation of the refrigerating fluid pressurevs. time of the multiple-stage refrigeration or cooling systemillustrated in FIG. 1.

FIG. 5 is a graphical representation of suction pressure settingsoptimized for preferred environmental temperature.

FIG. 6 is a graphical representation of when the "cut-in" and "cut-out"suction pressures are made equal.

FIG. 7 is an example, in tabulated form, illustrating how differentstages are turned-on in a multistage cooling system.

DESCRIPTION OF PREFERRED EMBODIMENT

The system of the invention, illustrated in FIG. 1, includes a pluralityof compressors with at least one compressor having variable speed, acondenser coil, at least one condenser fan, at least one expansion valveand evaporator coil, a plurality of system parameter sensing devices anda microprocessor based control system.

Referring now to FIG. 1, the refrigeration system capacitymicroprocessor controller 10 is shown disposed in a multiple-stagerefrigeration or cooling system 20 having a plurality of parallel pipedrefrigerant compressors 12, 14, 16, and 18 for discharging compressedpressurized refrigerant vapor through discharge line 22 to a condensercoil 25 where the pressurized refrigerant vapor is condensed to a liquidand then delivered to a receiver vessel 26. From the receiver 26, theliquid refrigerant flows through line 28 and through an expansion deviceor valve 30, typically a mechanical expansion valve responding to thetemperature in suction line 38 as sensed by temperature sensing device32. The temperature signal from sensor 32 is applied to valve 30 throughconductor 33 to initiate the expansion valve action. The liquidrefrigerant is injected through expansion device 30 into the evaporatorcoil 35 where the liquid refrigerant, encountering the low pressure ofthe evaporator coil, boils and evaporates thus absorbing heat from theevaporator coil. The hot vaporized refrigerant from the evaporator coilis drawn through suction line 38 to the inlet ports of the multiplecompressors 12-18. The number of parallel compressors running in thesystem varies according to the refrigerating or cooling system load. InFIG. 1, the compressors are shown as 12, 14, 16 and N, N being apositive integer. Compressors 14 and 18 are shown having fixed capacityand speed, compressor 12 having variable speed and compressor 16 havingan unloader 39. The system of this invention has at least one variablespeed compressor. It will be understood that any combination of fixedspeed, variable speed, and unloader-type compressors can be utilized forthe purpose of this invention.

A pressure detecting means 40, such as a transducer or a pressureswitch, is attached to the suction line 38. This pressure transducerdetermines the refrigerant vapor pressure within suction line 38 andgenerates an electrical signal representative of the measured pressure.The signal is applied through conductor 42 as an input to themicroprocessor controller 10. A second pressure transducer 164 isattached to the condenser coil 25, which determines the pressure insidethe condenser coil and generates an electrical signal representative ofthe measured pressure. The signal is applied through conductor 43 as aninput to the system capacity microprocessor controller. Alternately atemperature sensor or transducer 164a may be used in place of thepressure transducer 164. When a temperature transducer 164a is used inplace of the pressure transducer 164, it is attached at the receiver endof the condenser coil, which ensures that the temperature transducer isin contact with the liquid coolant in the condenser 25, thus ensuringprompt response to changes in the condensate temperature.

The microprocessor controller 10 illustrated in FIG. 1 has a pluralityof outputs corresponding to the number of the cooling stages, i.e., thenumber of parallel compressors and unloaders staged in the system.Unloader 39 is equivalent to another compressor in parallel.Accordingly, there are a corresponding "N" number of outputs from thesystem capacity microprocessor controller 10 labelled 1', 2', 3', 4',5', 6' and N'. The microprocessor controller output 2' is appliedthrough conductor 58 to the coil of a relay 44 which controls relayswitch contacts 45 for applying AC power via conductors 52 and 54 to thesecond compressor 14 for energizing the compressor when it is desired tocut the compressor into the system. Similarly, the 3' and N' outputs ofthe microprocessor controller are applied through conductors 60 and 62,respectively, to the coils of relays 48 and 50, respectively, forclosing switches 49 and 51, respectively, for applying AC electricalpower to the N compressors, respectively, for either turning on orturning off the compressors. The microprocessor controller output 1' isapplied through conductor 56 and carries an analog control signal whichvaries the speed of the variable speed compressor 12. The controllercircuit output 4' is applied through conductor 61 to the coil of a relay46 which controls relay switch contacts 47 which applies AC power toenergize the unloader 39 of compressor 16. The microprocessor controlleroutput 5', also an analog signal, is applied through conductor 63 andvaries the speed of the condenser fan 160. The controller circuit output6' is applied through conductor 67 to energize the condenser fan 162.

FIG. 2 illustrates a microprocessor-based control system for controllingthe refrigeration system illustrated in FIG. 1. However, it should beunderstood that an analog or digital control system may acheive similarresults. The control system of FIG. 2 includes a microprocessor, memoryfor storing a program, and input/output circuitry. Data is input intothe microprocessor controller 10 via keyboard 68, and a Display 70displays the data received from the microprocessor controller. Themicroprocessor controller 10, receives the values of system parameters,such a pressure and temperature via lines 42, 43, 43a and the like. FIG.2, as an example, illustrates two pressure sensors 40 and 164 and atemperature sensor 164a.

The pressure transducer 40 is sealingly inserted into the refrigerantvapor flow 65 in suction line tubing 38. Similarly pressure transducer164 is sealingly inserted into the condenser line 25. The temperaturesensor 164a may be used in place of the pressure transducer 164. Thepressure transducer 164 or the temperature transducer 164a may be anyconventional pressure or temperature detecting means for generating anelectrical signal representative of the corresponding pressure ortemperature within the condenser coil 25.

To fully understand the operation of the control system of FIG. 2, it isconsidered helpful to first understand the control parametersillustrated in the graph of FIG. 4. The graph of FIG. 4 depicts thesystem refrigerating fluid pressure vs. time wherein the system suctionpressure is represented by pressure trace 121. Referring to FIG. 4, aspeed increase suction pressure 134 and a speed decrease suctionpressure 136 are stored in the microprocessor controller. Themicroprocessor controller constantly monitors the system suctionpressure, and increases the speed of the variable speed compressor whenthe suction pressure represented by the trace 121 exceeds the pressurerepresented by the speed increase pressure 134 and decreases the speedof the variable speed compressor when the suction pressure falls belowthe speed decrease pressure 136. The variable speed compressor endeavorsto maintain the system suction pressure within the limits Δ P'.Similarly, a "cut-in" pressure 120 and a "cut-out" pressure 130 arestored in the microprocessor controller. When the suction pressureexceeds the "cut-in" pressure the microprocessor controller increasesthe speed of the variable speed compressor. If the variable speedcompressor is at its maximum speed and the suction pressure is below the"cut-in" setting, the microprocessor controller energizes a compressorstage in order to reduce the suction pressure back into the Δ P range.Similarly, when the suction pressure falls below the "cut-out" pressurethe variable speed compressor slows down until it reaches apredetermined minimum speed or until a low oil pressure warning signalis received. When a low oil pressure warning signal is received thespeed of the compressor is increased until the low oil warning isnulled. When the variable speed compressor cannot satisfy the systemsuction pressure reduction requirement, the microprocessor controllerdeenergizes a compressor stage and increases the speed of the variablespeed compressor, if necessary, until the suction pressure is returnedto the Δ P' range.

An example of how the multiple-stage compressor system of the inventionchanges capacity is illustrated in FIG. 7. Referring to FIG. 7, C1, C2,and C3 represents compressors in the system. C1 is a variable speedcompressor. U1 and U2 are unloaders. A "0" represents a relay in adeenergized state. A "1" represents a relay in an energized state."Stage 0" is the system shut-down condition when all relays aredeenergized. In stage 1, the variable speed compressor C1, whichpreferably has the largest capacity in the system, is energized. At"stage 0" the evaporation suction pressure is at a higher pressure thanthe "cut-in" pressure 120 as illustrated in FIG. 4. As the speed of thecompressor of C1 is increased, suction pressure will fall towards the"cut-in" pressure 120. The microprocessor controller will increase thespeed of the compressor C1 until the suction pressure falls below thespeed increase pressure 134. The compressor C1 will be controlled atsuch a speed as to maintain the suction pressure within the range of ΔP'. If the suction pressure falls below the speed decrease pressure 136,compressor C1 is slowed down. If the suction pressure rises above thespeed increase pressure 134, the compressor C1 is driven faster. Whencompressor C1 reaches maximum speed and the suction pressure remainsabove the "cut-in" pressure 120 stage 2 turns on, i.e., compressor C2 isenergized. If the suction pressure falls below the speed increasesuction pressure 134, the microprocessor controller reduces the speed ofcompressor C1 to maintain the suction pressure within the Δ P' range inthe same way as in stage 1. When the suction pressure rises above the"cut-in" pressure stage 3 comes into service using the unloader U1. Whenthe unloader U1 brings the suction pressure down below thespeed-increase pressure 134 the variable speed compressor C1 continuesto hold the suction pressure within the Δ P' range as in the previousstages. Similarly, as compressor load demand increase "stages 4" and"stages 5" are brought into service and at each stage compressor C1 isused to fine control the suction pressure. When the evaporator suctionpressure falls below the "cut-out" pressure 130 and the variable speedcompressor is at its minimum speed or the oil pressure falls below itsminimum setting the microprocessor controller changes down to the nextlower stage. The variable speed compressor already at minimum speed willincrease speed to maintain the suction pressure with the ΔP' range.

The microprocessor controller 10 is programmed to have a "cut-in" and a"cut-out" time delay routine. When the microprocessor controller electsto change the on/off state of a compressor or an unloader, a timer isset and a programmed delay precedes the selected change of state.

The time delay routine remembers the length of the respective timedelays, Δ T and Δ t. As an example, referring to FIG. 4, if Δ T is setto five (5) minutes, and Δ t is set to five (5) seconds, when suctionpressure trace 121 rises above the "cut-in" pressure 120 at point 122,the five (5) minute Δ T period begins. However, if suction pressuretrace 121 drops back below "cut-in" pressure 120 prior to reaching point124 i.e. less than Δ T, the "cut-in" signal will cease, disabling"cut-in" time delay, and demands no additional capacity from thecompressor unloader combination. However, the next time the suctionpressure 121 exceeds the "cut-in" pressure, "cut-in" time delay willagain be energized and will produce a delayed "cut-in" signal after onlythree (3) minutes (the balance of Δ T left over from the last Δ Tperiod), thereby energizing or turning on the next compressor insequence. When suction pressure trace 121 falls below the "cut-out"pressure 130 at point 126, the five (5) second Δ t period begins.However, when the suction pressure 121 increases and rises above"cut-out" pressure 130 after only three (3) seconds, the "cut-out"signal will cease and disable "cut-out" time delay. The next time thepressure trace 121 decreases and falls below the "cut-out" pressure,"cut-out" time delay is enabled and produces a delayed "cut-out" signalafter only two (2) seconds (the balance of Δ t left over from the last Δt period) and "cut-out" or turn off the compressor which has run thelongest. The time delays are applied to unloaders in the same manner asif they were compressors.

In a refrigeration system having equal capacity compressors themicroprocessor controller may select the compressor which has beenrunning the longest when it is required to "cut-out" a compressor, orselect the compressor which has been idle the longest when it isrequired to "cut-in" a compressor. The selection sequence mentionedabove shares the turn-on and turn-off operations between compressorsimproving system reliability and increasing compressor life expectancy.

In a refrigeration system having unequal capacity compressors, themicroprocessor controller 10 will energize or deenergize, i.e., turn onor turn off the next compressor as described above until the combinationof stages has an operating capacity closest to the system load or systemcapacity demand, thus, causing the system suction pressure to return tothe previously established Δ P range as described hereinabove andillustrated in FIG. 4. As an example, consider a refrigeration systemhaving three unequal capacity compressors 12, 14 and 16. Furtherconsider that the compressors are rated at capacities 1, 2 and 4horsepower (HP), respectively. The microprocessor controller 10 willselect and provide increased or decreased compressor horsepower capacityin discrete increments or combinations to match the system capacitydemand. Assuming the above described ratings of 1, 2 and 4 HP forcompressors 12, 14 and 16, the various possible combinations of thesecompressors will provide capacities of 1, 2, 3, 4, 5, 6 and 7 HP inresponse to changing system capacity demand. The number of combinationsfor multiple unequal compressors i.e. the number of compressor stageswill always be larger than the number of compressors in the system.

Referring back to FIG. 1, the refrigeration capacity control systemdisclosed herein may also be utilized in controlling a multiple-stagerefrigeration or cooling system having multi-cylinder compressors thatare staged by controlling the compression of a plurality of compressorcylinders using conventional control valves by having microprocessorcontroller 10 output control the utilization of the cooling stages bycontrolling the loading and unloading of cylinders used by thecompressors in the system. In addition, it is important to understandthat while the system described in FIGS. 1, 2 and 4 uses a separate timedelay after determination of the reaching of the established "cut-in" or"cut-out" pressures, only a single time delay is necessary to enable theselection of successive cooling stages utilizing a single selected"cut-in" system pressure and a single selected "cut-out" systempressure. Further, the Δ P pressure differential between "cut-in"pressure 120 and "cut-out" pressure 130 illustrated in FIG. 4 may be setlarge or small, depending on the system design and the system operatingpressure. In certain systems, the Δ P could be set at zero, with the"cut-in" and "cut-out" pressures being established at the same value.

Referring to FIG. 4, the suction pressure ranges Δ P and Δ P' may bevaried. Further, Δ P may be made equal to Δ P' or Δ P' may range higherthan the "cut-in" pressure 120 or may range lower than the "cut-out"pressure 130. Additionally, all pressure settings illustrated in FIG. 4may be made equal thus yielding a single pressure line as illustrated inFIG. 6. The single line illustrated in FIG. 6 results when Δ P and Δ P'are both equal to zero.

In a further embodiment the temperature of an environment is monitoredby the microprocessor controller. Refer now to FIG. 5. In the event theenvironment is in a too cold region 520 the microprocessor controller isprogrammed to raise the evaporator suction pressure limits asillustrated. Conversely, in the event the environment is too warm, themicroprocessor controller shifts the evaporator suction pressure limitsto a lower value.

When a plurality of evaporator coils are used to cool a plurality offixtures the temperature of the coldest fixture is used for controllingthe refrigeration system. Other fixtures may be maintained at a highertemperature by adjusting a flow control valve fitted to the evaporatorcoil cooling that particular fixture.

FIG. 3 illustrates a flow diagram for the system control sequence of thesystem described in FIG. 1. Section 320 observes the compressor "cut-in"pressure 120 and starts the delay timer if it is high. If the "cut-in"pressure is high, the timer has elapsed and the variable speedcompressor is at maximum speed, system capacity is increased byenergizing a fixed staged compressor. Section 322 follows a similarsequence with respect to the compressor "cut-out" pressure 130 anddeenergizes a fixed compressor stage when required. Section 324 monitorsthe speed increase pressure 134 and increases the speed of the variablespeed compressor when necessary. Section 326 decreases the speed of thevariable speed compressor when the speed decrease pressure 136 is low. Alow oil pressure check is made at section 328, ensuring the speed of thevariable speed compressor is increased when the oil pressure is low.Having detected a low oil pressure the flow path is taken directly tosection 322 where a decrease capacity command turns off a compressorstage. The flow path then proceeds to section 324 and will increase thespeed of the variable speed compressor to ensure a safe oil pressure.Section 330 monitors the temperature of the environment being cooled andraises or lowers the suction pressure ranges Δ P and Δ P' to provide thedesired environment temperature conditions.

In a further embodiment, control of the refrigeration system is obtainedby using condenser coil fans 160 and 162. Referring to FIG. 4, suctionpressure is replaced by condenser pressure and pressure trace 121 nowrepresents the condenser pressure. Condenser pressure is maintainedwithin predetermined pressure limits over the range of Δ P' by varyingthe speed of the variable speed fan. When the condenser pressureincreases above the predetermined "cut-in" pressure or falls below thepredetermined "cut-out" pressure fan stages are turned-on or turned-off,respectively, bringing the condenser pressure into the Δ P range. Thevariable speed fan is used continually to optimise the condenserpressure. Condensate temperature may be used in place of condenserpressure. Suction pressure becomes condensate temperature and trace 121is now a trace of temperature variation. The temperature controlprocedure is the same as for pressure control. Time delays Δ T and Δ twill be used preceding the turning on and turning off of fans. Differentdelays will be selected depending on whether temperature control orpressure control is being used and delays will also be varied accordingto system size and load conditions. The control procedure describedabove will increase the coefficient of performance when theoptimum-value of pressure or temperature have been determined for thesystem.

While the above description of the preferred embodiments has been madewith particular reference to multiple-stage refrigeration system usingparallel staged compressors or staged multiple-cylinder compressors, itwill be appreciated that the capacity controlling method and apparatusdescribed herein may be utilized in controlling the capacity of amultiple-stage cooling system such as air-conditioning systems utilizingchilled water and the like, by controlling cooling stages in thosesystems by controlling utilization of water circulating pumps orcontrolling the utilization of vanes in centrifugal pumps and the like.Accordingly, numerous variations and modifications may be made in themethods and structure herein described without departing from thepresent invention.

What is claimed is:
 1. A refrigeration system having in a closed loopconnection a compressor for compressing a refrigerant, condensing meansfor condensing the compressed refrigerant from the compressor anddischarging the condensed refrigerant at a discharge pressure into anoutlet of said condensing means, said system further comprising:(a) apre-selected number of condenser fans, at least one condenser fan beingadapted to operate between a maximum and a minimum operating speed; (b)discharge pressure selection means for establishing a first and seconddischarge pressure range, each said discharge pressure range having anupper and a lower limit, the first discharge pressure range beingassociated with the variable speed fan and the second discharge pressurerange being associated with the remaining fans; (c) detection means forsensing the discharge pressure and for cooperating with the dischargepressure selection means to provide an increase speed signal when thedischarge pressure exceeds the upper limit of the first dischargepressure range, a decrease speed signal when the discharge pressurefalls below the lower limit of the first discharge pressure range, anincrease capacity signal when the discharge pressure exceeds the upperlimit of the second discharge pressure range and when the variable speedfan is operating at the maximum speed, and a decrease capacity signalwhen the discharge pressure falls below the lower limit of the secondpressure range and when the variable speed fan is operating at theminimum speed; and (d) selection means for receiving the increase speedand decrease speed signals and in respective response thereto increasingand decreasing the speed of the variable speed fan, and for receivingthe increase capacity and decrease capacity signals and in respectiveresponse thereto energizing and deenergizing a fan of the refrigerationcompressor system.
 2. A refrigeration system having in a closed loopconnection at least one compressor for compressing a refrigerant,condensing means for condensing the compressed refrigerant from thecompressor and discharging the condensed refrigerant into an outlet ofsaid condensing means, said system further comprising:(a) a pre-selectednumber of condenser fans, at least one condenser fan being adapted tooperate between a maximum and a minimum operating speed; (b) temperatureselection means for establishing a first and a second temperature range,each said temperature range having an upper and a lower limit, whereinthe first temperature range being associated with the variable speed fanand the second temperature range being associated with the remainingfan; (c) detection means for sensing the temperature of the refrigerantat the outlet of the condensing means and for cooperating with thedischarge temperature selection means to provide an increase speedsignal when the refrigerant temperature exceeds the upper limit of thefirst temperature range, a decrease speed signal when the refrigeranttemperature falls below the lower limit of the first temperature range,an increase capacity signal when the refrigerant temperature exceeds theupper limit of the second temperature range and when the variable speedfan is operating at the maximum speed, and a decrease capacity signalwhen the temperature falls below the lower limit of the secondtemperature range and when the variable speed fan is operating at theminimum speed; and (d) selection means for receiving the increase speedand decrease speed signals and in respective response thereto increasingand decreasing the speed of the variable speed fan, and for receivingthe increase capacity and decrease capacity signals and in respectiveresponse thereto energizing and deenergizing a fan of the refrigerationcompressor system.
 3. A refrigeration system comprising:(a) apredetermined number of commonly piped compressors having a commonsuction pressure for compressing a refrigerant, and wherein at least onecompressor being adapted to operate between a maximum and a minimumoperating speed; (b) condensing means for condensing the compressedrefrigerant from the compressors and for discharging the condensedrefrigerant at a discharge pressure into an outlet of the condensingmeans; (c) a predetermined number of fans placed near the condensingmeans for aiding in the condensation of said compressed refrigerantwherein at least one fan being adapted to operate between a maximum anda minimum operating speed; (d) pressure selection means for establishinga first and a second suction pressure range, each said pressure rangehaving an upper and lower limit, the first pressure range beingassociated with the variable speed compressor and the second pressurerange being associated with the remaining compressors; (e) detectionmeans for sensing the suction pressure and for cooperating with thepressure selection means to provide an increase speed signal when thesuction pressure exceeds the upper limit of the first pressure range, adecrease speed signal when the suction pressure falls below the lowerlimit of the first pressure range, an increase capacity signal when thesuction pressure exceeds the upper limit of the second pressure rangeand that the variable speed compressor is operating at the maximumspeed, and a decrease capacity signal when the suction pressure fallsbelow the lower limit of the second pressure range and that the variablespeed compressor is operating at the minimum speed; (f) selection meansfor receiving the increase speed and decrease speed signals and inrespective response thereto increasing and decreasing the speed of thevariable speed compressor, and for receiving the increase capacity anddecrease capacity signals and in respective response thereto energizingand deenergizing a compressor of the refrigeration compressor system;(g) discharge pressure selection means for establishing a first andsecond discharge pressure range, each said discharge pressure rangehaving an upper and a lower limit, wherein the first discharge pressurerange being associated with the variable speed fan and the seconddischarge pressure range being associated with the remaining fans; (h)detection means for sensing the discharge pressure and for cooperatingwith the discharge pressure selection means to provide an increase speedsignal when the discharge pressure exceeds the upper limit of the firstdischarge pressure range, a decrease speed signal when the dischargepressure falls below the lower limit of the first discharge pressurerange, an increase capacity signal when the discharge pressure exceedsthe upper limit of the second discharge pressure range and when thevariable speed fan is operating at the maximum speed, and a decreasecapacity signal when the discharge pressure falls below the lower limitof the second pressure range and when the variable speed fan isoperating at the minimum speed; and (i) selection means for receivingthe increase speed and decrease speed signals and in respective responsethereto increasing and decreasing the speed of the variable speed fan,and for receiving the increase capacity and decrease capacity signalsand in respective response thereto energizing and deenergizing a fan ofthe refrigeration compressor system.
 4. A refrigeration systemcomprising:(a) a predetermined number of commonly piped compressorshaving a common suction pressure for compressing a refrigerant, andwherein at least one compressor being adapted to operate between amaximum and a minimum operating speed; (b) condensing means forcondensing the compressed refrigerant from the compressors and fordischarging the condensed refrigerant into an outlet of the condensingmeans; (c) a predetermined number of fans placed near the condensingmeans for aiding in the condensation of said compressed refrigerantwherein at least one fan being adapted to operate between a maximum anda minimum operating speed; (d) pressure selection means for establishinga first and a second suction pressure range, each said pressure rangehaving an upper and a lower limit, the first pressure range beingassociated with the variable speed compressor and the second pressurerange being associated with the remaining compressors; (e) detectionmeans for sensing the suction pressure and for cooperating with thepressure selection means to provide an increase speed signal when thesuction pressure exceeds the upper limit of the first pressure range, adecrease speed signal when the suction pressure falls below the lowerlimit of the first pressure range, an increase capacity signal when thesuction pressure exceeds the upper limit of the second pressure rangeand that the variable speed compressor is operating at the maximumspeed, and a decrease capacity signal when the suction pressure fallsbelow the lower limit of the second pressure range and that the variablespeed compressor is operating at the minimum speed; (f) selection meansfor receiving the increase speed and decrease speed signals and inrespective response thereto increasing and decreasing the speed of thevariable speed compressor, and for receiving the increase capacity anddecrease capacity signals and in respective response thereto energizingand deenergizing a compressor of the refrigeration compressor system;(g) temperature selection means for establishing a first and a secondtemperature range, each said temperature range having an upper and alower limit, the first pressure range being associated with the variablespeed fan and the second temperature range being associated with theremaining fans; (h) detection means for sensing the temperature of thedischarged refrigerant and for cooperating with the temperatureselection means to provide an increase speed signal when the refrigeranttemperature exceeds the upper limit of the first temperature range, adecrease speed signal when the refrigerant temperature falls below thelower limit of the first temperature range, an increase capacity signalwhen the refrigerant temperature exceeds the upper limit of the secondtemperature range and when the variable speed fan is operating at themaximum speed, and a decrease capacity signal when the refrigeranttemperature falls below the lower limit of the second pressure range andwhen the variable speed fan is operating at the minimum speed; and (i)selection means for receiving the increase speed and decrease speedsignals and in respective response thereto increasing and decreasing thespeed of the variable speed fan, and for receiving the increase capacityand decrease capacity signals and in respective response theretoenergizing and deenergizing a fan of the refrigeration compressorsystem.
 5. A method of controlling the capacity of a refrigerationsystem having a number of commonly piped compressors compressing arefrigerant, said compressors having a common suction pressure and atleast one of the compressors being of variable speed type, a condensermeans for condensing the compressed refrigerant at a discharge pressure,a cooling means containing a plurality of fans placed near the condensermeans wherein at least one of the fans being of variable speed type,said method comprising the steps of:(a) establishing a suction pressurerange having an upper and a lower limit; (b) determining the suctionpressure; (c) providing an increase speed signal when the suctionpressure exceeds the upper limit of the pressure range and increasingthe speed of the variable speed compressor in response thereto, andproviding a decrease speed signal when the suction pressure falls belowthe lower limit of the pressure range and decreasing the speed of thevariable speed compressor in response thereto; (d) providing an increasecapacity signal when the suction pressure exceeds the upper limit of thepressure range and that the variable speed compressor is operating atthe maximum speed and energizing a compressor of the refrigerationsystem in response thereto, and providing a decrease capacity signalwhen the suction pressure falls below the lower limit of the pressurerange and that the variable speed compressor is at the minimum speed andde-energizing a compressor of the multistage compressor system inresponse thereto; (e) establishing a discharge pressure range having anupper and a lower limit; (f) determining the discharge pressure; (g)providing a fan increase speed signal when the discharge pressureexceeds the upper limit of the discharge pressure range and increasingthe speed of the variable speed fan in response thereto, and providing afan decrease speed signal when the discharge pressure falls below thelower limit of the discharge pressure range and decreasing the speed ofthe variable speed fan in response thereto; and (h) providing a fanincrease capacity signal when the discharge pressure exceeds the upperlimit of the discharge pressure range and that the variable speed fan isoperating at the maximum speed and energizing a fan of the refrigerationsystem in response thereto, and providing a fan decrease capacity signalwhen the discharge pressure falls below the lower limit of the dischargepressure range and that the variable speed fan is at the minimum speedand de-energizing a fan of the system in response thereto.
 6. Apparatusfor controlling the capacity of a refrigeration system having onevariable speed compressor adapted to operate at variable speeds betweena maximum and minimum speed and a plurality of equal capacitycompressors, said compressors also having a common suction pressure,said system comprising:(a) a pressure selecting means for establishingan operating suction pressure range; (b) a detection means for sensingthe suction pressure and cooperating with said pressure selecting meansto generate an increase speed signal when the suction pressure exceedsthe suction pressure range, a decrease speed signal when the suctionpressure falls below the suction pressure range, an increase capacitysignal when the suction pressure exceeds the suction pressure range andthat the variable speed compressor is at the maximum speed, and adecrease capacity signal when the suction pressure falls below thesuction pressure range and that the variable speed compressor is at theminimum speed; and (c) selection means for receiving the increase speedsignal and decrease speed signals and in respective response theretoincreasing and decreasing the speed of the variable speed compressor,and for receiving the increase capacity and decrease capacity signal andin respective response thereto selectively energizing and de-energizingcompressors from said plurality of compressors so that the compressorthat has been turned-off the longest is the first to be energized andthe compressor that has been turned-on the longest is the first to bede-energized.
 7. Apparatus for controlling the capacity of arefrigeration system having variable speed compressors adapted tooperate at variable speeds between a maximum and a minimum operatingspeed and a plurality of unequal capacity compressors, said compressorsalso having a common suction pressure, said system comprising:(a) apressure selecting means for establishing and operating suction pressurerange; (b) a detection means for sensing the suction pressure andcooperating with said pressure selecting means to generate an increasespeed signal when the suction pressure exceeds the suction pressurerange, a decrease speed signal when the suction pressure falls below thesuction pressure range, an increase capacity signal when the suctionpressure exceeds the suction pressure range and that the variable speedcompressor is at the maximum speed, and a decrease capacity signal whenthe suction pressure falls below the suction pressure range and that thevariable speed compressor is at the minimum speed; and (c) selectionmeans for receiving the increase speed signal and decrease speed signalsand in respective response thereto increasing and decreasing the speedof the variable speed compressor, and for receiving the increasecapacity and decrease capacity signal and in respective response theretoenergizing and de-energizing a compressor from said plurality ofcompressors in a way that most closely matches the system compressorcapacity with the refrigeration load of the refrigeration system.